This invention pertains to apparatus for monitoring high-speed (subnanosecond) phenomena, through modulating, streaking, and recording over time the path of a light beam. More particularly, it pertains to a portion of such apparatus which employs, in a crystal, an electro-optically induced traveling lens which is caused to interact with multiple passes of the beam of light as the same travels through the crystal.
There are many applications, in modern physics and industry, where it is important to be able to detect, monitor and record accurately extremely high-speed events. Over the years, progressively more sophisticated techniques and apparatus have been developed for increasing time resolution, thus to enable "capture" of such events. One of the most current of these techniques involves the voltage-inducement, in certain optically transparent crystals, of a synthesized traveling lens, which, as it travels, causes a short-duration burst of light, as from a laser, to be swept along a defined path. The swept light beam is projected onto an optically sensitive film, and information, as from a high-speed event, is used to modulate the intensity of the beam so as to produce a readable indication in the filmed event. As is well known, the voltage which exists across such a crystal affects its refractive index. By using a substantially parabolic, short-duration voltage pulse, which pulse travels in conductors distributed along the opposite sides of such a crystal, there results a traveling refractive index effect (traveling nearly at the speed of light in the crystal) that has been shown to be effective in sweeping or streaking a light beam. In effect, such a pulse induces in the crystal a synthesized traveling lens.
As a result of ongoing concern for improving even further the time resolution obtainable with such a technique, we have devised a significant improvement in such apparatus, which improvement appreciably increases the effective bandwidth of such apparatus, and greatly improves time resolution. In particular, proposed by the present invention is a modification wherein multiple cross reflections for a light beam are created in an elongated crystal. These reflections are coordinated with the traveling lens, so that each coherent wave front in the beam not only traverses the crystal multiple times, but also for each crossing passes through substantially the same "lens portion". As a consequence of this kind of activity, each wave front in a beam experiences what might be thought of as a multiplied lens effect--namely one which is multipled by the number of passes through the traveling lens. Thus, focusing is greatly enhanced, and a significantly higher time resolution is possible, which means that significantly shorter-duration events can be monitored accurately. Put another way, enhanced focusing increases the number of resolvable points in a recorded trace of a swept light beam.
By featuring the multiple-reflection activity just outlined, other important advantages are attained. For example, it is a relatively simple matter to couple, endo, plural elongated crystals, thus to increase the number of cross reflections which are possible--thereby further increasing the lensing effect. The sizes of such crystals, and the geometry of the overall apparatus, are easily controlled so that the regions of abutment or joinder between adjacent crystals produce no optical errors. Further, the multiple cross-reflection technique tends to minimize the effect on the finally streaked light beam of local imperfections that exist within the body of a crystal.
Another important advantage of the multiple cross-reflection technique is that the overall electrical impedance of the assembly formed by such a crystal, and by the conductors sandwiched therewith, can be maintained at a maximum level. Further explaining, it would be possible, without using multiple cross-reflections, to achieve enhanced focusing simply by increasing the single-pass-through distance in a crystal that a light beam travels before emerging. However, by enlarging crystal depth to accomplish this, the impedance of a crystal-conductor assembly would be extremely low, and this could present a serious problem in "loading" excessively whatever source is used to generate the required voltage pulse. Such loading is minimized by minimizing crystal dimension in the direction of light travel, while at the same time providing for multiple passes through the crystal, thus to increase the "effective" distance through which a light beam travels in the crystal.
One embodiment of the proposed invention is described wherein reflections are produced through the actions of opposing mirrors disposed adjacent a pair of outside opposite faces in a crystal. In another modification which is mentioned briefly herein, advantage is taken of what is known as the critical reflection angle in the crystal--namely that angle of total internal reflection--where outside mirrored surfaces are not required.
These and other objects and advantages obtained by the present invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawings.